On Sun, 10 Sep 2006 01:27:08 -0700, Richard Clark
wrote:

On Sun, 10 Sep 2006 08:02:14 GMT, "Alan" wrote:

I guessed they might be but does anybody use a smaller antenna as a
compromise?

Hi Alan,

Sure. So's the signal (a compromise). When you get into fractional
wavelength sizes (and we're talking on the order of less than 5% of
wavelength), you quickly get into fractional performance. It has to
do with the comparison of the radiation resistance which at 5% is 2
Ohms. This is 25:1 mismatch, and the matching components are going to
have to present less than 2 Ohms in themselves (not forgetting all the
connections that support the circulating currents) just to limit your
loss to 3dB.

Richard,

May I expand on a couple of your points.

Amateurs are very focused on VSWR.

Additionally, they hate narrowband mobile antennas.

Radiation resistance of short verticals on low HF bands is very low,
whether or not they are base loaded, centre loaded, top loaded,
capacitively loaded, or continuously loaded.

Even when the resistance of the radiating car body and earth coupling
are added in, the direct base impedance of a short loaded resonated
vertical mounted on a car is well below 50 ohms.

If the loading coil is made sufficiently lossy, then the feedpoint
resistance can be raised sufficiently to satisfy a "reasonable" VSWR
spec, in fact the better (lower) the VSWR spec for direct feeding, the
lossier the loading coil need to be.

The upside of this is that the bandwidth is expanded, isn't that good!

We have delivered on the two most important criteria!

Well, these things (simplified feed arrangement for tolerable VSWR,
better bandwidth) are obtained at the expense of efficiency.

If you view the world through an VSWR meter, you will be pleased with
short direct fed verticals for cars.

If you want efficiency, then it will almost certainly require a base
matching network.

With a suitable base matching network and low loss loading coils, you
may still achieve reasonable efficiency in a shorter vertical, though
low loss coils are not low profile! Secondly, commercial manufacturers
of low loss verticals skip straight to long antennas.

Like Richard, I am sure lots of people will be happy that anything
"works".

Owen
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